The invention relates to the field of audio receivers, and in particular to audio receivers that employ noise suppression to compensate for noise in the reception signal.
FM radio transmitters transmit a stereo multiplex signal, which is formed from an audio central signal (also referred to as a mono signal) with a frequency up to 15 KHz, a stereo pilot tone with a frequency of 19 KHz, and a stereo signal with a frequency of 23 KHz to 53 KHz.
The stereo multiplex signal is received by a stereo receiver and digitized by an analog-digital converter to provide a digital stereo multiplex signal. The digital stereo multiplex signal is processed in a stereo decoder to produce a (L−R) signal and a (L+R) signal. By forming sum and difference signals from the (L−R) signal and the (L+R) signal, a stereo matrix calculates the audio signal for the left channel, often referred to as the L signal, and the audio signal for the right channel, often referred to as the R signal.
In contrast to a fixed stereo radio receiver, reception on a mobile stereo radio (e.g., a receiver built into a motor vehicle) is frequently disturbed by various, sometimes even inadequate, reception conditions due to the topography, multi-path reception, or radio shadows. In addition to these reception problems caused by the topography of the terrain, the electrical system (e.g., the ignition system) of a motor vehicle also causes noise pulses in the reception signal of the stereo radio receiver.
Known techniques for suppressing reception problems are controlling the channel separation (i.e., the continuous blending of stereo to mono reproduction), immediate switching from stereo to mono reproduction, use of low-pass filters, and noise blanking in which the reception signal is muted for a presettable time.
Noise blanking at the level of the stereo multiplex signal proves to be especially effective in suppressing noise caused by multi-path reception because it prevents noise pulses from propagating to the audio level. However, noise blanking has the disadvantage that gaps are produced in the reception signal.
Switching from stereo to mono reproduction is especially suitable with declining reception field strengths (i.e., with increasing noise components) to improve reception quality. The disadvantage of switching from stereo to mono reproduction is that stereophonic quality becomes unavailable.
Needle- or pulse-shaped noises caused for example by the ignition system of a motor vehicle can be largely suppressed by a low-pass filter. However, the total acoustic pattern is adversely affected by the filtering.
Techniques are also known that provide for interpolation of scanning values in the stereo multiplex signal to suppress individual noise pulses. However, these techniques require a relatively high scanning rate, leading to a high manufacturing cost.
Therefore, there is a need to suppress noise in the reception signal of a stereo radio receiver, as well as a stereo radio receiver, so that stereo reproduction that lasts as long as possible is obtained at relatively low manufacturing cost when there is noise in the reception signal.